Recent Results and Perspectives for Precision Astrometry and Photometry with Adaptive Optics

Large ground-based telescopes equipped with adaptive optics (AO) systems have ushered in a new era of high-resolution infrared photometry and astrometry. Relative astrometric accuracies of <0.2 mas have already been demonstrated from infrared images with spatial resolutions of 55-95 mas resolution over 10-20'' fields of view. Relative photometric accuracies of 3% and absolute photometric accuracies of 5%-20% are also possible. I will review improvements and current limitations in astrometry and photometry with adaptive optics of crowded stellar fields. These capabilities enable experiments such as measuring orbits for brown dwarfs and exoplanets, studying our Galaxy's supermassive black hole and its environment, and identifying individual stars in young star clusters, which can be used test the universality of the initial mass function.Comment: SPIE Conference Proceedin

Clarifying our View of Star Formation in Massive Young Clusters with Adaptive Optics

Observations of massive (> 10^4 M_⊙), young (<10 Myr) star clusters within our Galaxy allow us to fully sample the upper end of the initial mass function within a single star formation event. Such clusters also reside in a range of environments including the Galactic disk, the Galactic center region, and immediately surrounding the supermassive black hole in our Galactic nucleus. However, studies of these clusters are limited by crowding in the dense cores, strong and variable visible extinction, and confusion between cluster members and contaminating field stars. Using Keck laser-guided adaptive optics observations, we obtain high-resolution images and high-precision proper motions to both identify individual cluster members and investigate the kinematic properties of such clusters. As we build up complete proper motion data sets for several massive young clusters, our multi-color near-infrared photometry will yield precise mass functions that can be compared to search for environmental dependencies

The Chemical Evolution of the Galactic Bulge

This science white paper addresses the issue of discovering the chemical evolution of the Galactic bulge, from which we may learn the initial mass function at the time of the formation of the bulge, the timescale for the initial burst of star formation, any evidence supporting an extended era of star formation, evidence of very early mergers of massive subcomponents, and the fraction of its mass that was contributed by late mergers. A further immediate problem concerns the composition of dwarfs measured from microlensing events versus the abundance scale measured from giants. A companion White Paper (Clarkson & Rich) addresses a set of bulge science questions that require observations at very high angular resolution

On the luminosity distance and the epoch of acceleration

Standard cosmological models based on general relativity (GR) with dark energy predict that the Universe underwent a transition from decelerating to accelerating expansion at a moderate redshift $z_{acc} \sim 0.7$. Clearly, it is of great interest to directly measure this transition in a model-independent way, without the assumption that GR is the correct theory of gravity. We explore to what extent supernova (SN) luminosity distance measurements provide evidence for such a transition: we show that, contrary to intuition, the well-known "turnover" in the SN distance residuals $\Delta\mu$ relative to an empty (Milne) model does not give firm evidence for such a transition within the redshift range spanned by SN data. The observed turnover in that diagram is predominantly due to the negative curvature in the Milne model, {\em not} the deceleration predicted by $\Lambda$CDM and relatives. We show that there are several advantages in plotting distance residuals against a flat, non-accelerating model $(w = -1/3)$, and also remapping the $z-$axis to $u = \ln(1+z)$; we outline a number of useful and intuitive properties of this presentation. We conclude that there are significant complementarities between SNe and baryon acoustic oscillations (BAOs): SNe offer high precision at low redshifts and give good constraints on the net {\em amount} of acceleration since $z \sim 0.7$, but are weak at constraining $z_{acc}$; while radial BAO measurements are probably superior for placing direct constraints on $z_{acc}$.Comment: Latex, 13 pages, 7 figures. Accepted by MNRAS. For the busy reader, Figs 4 and 6 are the main result

Stellar Proper Motions in the Galactic Bulge from deep HST ACS/WFC Photometry

We present stellar proper motions in the Galactic bulge from the Sagittarius Window Eclipsing Extrasolar Search (SWEEPS) project using ACS/WFC on HST. Proper motions are extracted for more than 180,000 objects, with >81,000 measured to accuracy better than 0.3 mas/yr in both coordinates. We report several results based on these measurements: 1. Kinematic separation of bulge from disk allows a sample of >15,000 bulge objects to be extracted based on >6-sigma detections of proper motion, with <0.2% contamination from the disk. This includes the first detection of a candidate bulge Blue Straggler population. 2. Armed with a photometric distance modulus on a star by star basis, and using the large number of stars with high-quality proper motion measurements to overcome intrinsic scatter, we dissect the kinematic properties of the bulge as a function of distance along the line of sight. This allows us to extract the stellar circular speed curve from proper motions alone, which we compare with the circular speed curve obtained from radial velocities. 3. We trace the variation of the {l,b} velocity ellipse as a function of depth. 4. Finally, we use the density-weighted {l,b} proper motion ellipse produced from the tracer stars to assess the kinematic membership of the sixteen transiting planet candidates discovered in the Sagittarius Window; the kinematic distribution of the planet candidates is consistent with that of the disk and bulge stellar populations.Comment: 71 pages, 30 figures, ApJ Accepte